The lack of a growth-promoting effect of orally administered bovine somatotropin in the rat body-weight-gain bioassay

Bovine somatotropin (bSt) was given either orally or subcutaneously to groups of female hypophysectomized rats daily for 9 days. Ten rats per dose group were given oral dosages of 0 (buffered-water vehicle control), 40, 400, 2000, and 4000 μg of bSt per day. Similar groups of ten rats each received subcutaneous doses of 0 (buffered-water vehicle control), 15, 30, and 60 μg of bSt per day. Rats were weighed daily to observe their body-weight gain, which is a measure of the biological activity of bSt in the hypophysectomized rat. At study termination, serum of treated rats was monitored for the presence of bSt and antibody to bSt. Bovine somatotropin was detected in the serum of the subcutaneously treated rats, but not in those rats treated orally. Of 18 rats treated subcutaneously with bSt, 14 developed antibodies to bSt, whereas of 38 rats treated orally with bSt, 11 developed antibodies. Subcutaneously treated rats grew in a dose-related manner as expected in this assay. Orally administered bSt failed to elicit a growth response at any dose in this sensitive bioassay system. The data suggest that neither bSt nor growth-promoting fragments of bSt are absorbed after oral administration of doses up to 40,000 μg/kg/day in the hypophysectomized rat.